I know you so well… Bu I also know the solution:
Update: 21/05/2013
Updating the kernel to 3.3.6 as suggested by Michael in #91, the system seems stable, no freezes yet in 5-6 hrs of using and watching videos for couple of hrs.
Update: 12/06/2013
This bug is about a complete system lock-up. No other TTYs can be started. Mouse doesn’t move. Keyboard doesn’t react. The only solution is hard reset.
Seems to happen on a variety of configurations, including nVidia, ATI and Intel graphics.
If you are still able to TTY or SSH into other sessions, move the mouse or use the keyboard, file another bug.
https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/993187
Update: 02/12/2013
The problem persisted for long time. Until a month ago… So I suspect the problem was about the ram. connector/slot of the laptop is damaged. 1 month ago I changed the RAM module to the other RAM slot. So far, great. Not even one crash. 😉 great!